High density polyethylene (PE) resins are increasingly being used for the manufacture of pipes and there is a continued need for the development of PE resins having increased resistance to stress cracking in order to extend the long-term durability of pipes produced therefrom.
Field experience has shown that pipe failures are often the result of slow crack growth and/or failure caused by sudden impact by a heavy load. As a result, slow crack growth (SCG) resistance and rapid crack propagation (RCP) tests have been developed and are used to differentiate performance of PE pipe resins. SCG resistance is determined using the so-called PENT (Pennsylvania Notched Tensile) test. The latter test was developed by Professor Brown at Pennsylvania University as a small scale laboratory test and has now been adopted as ASTM F 1473-94. RCP is determined on extruded pipe following the procedure of ISO 13477 or ISO 13478 or on a smaller scale using the Charpy Impact Test (ASTM F 2231-02).
For successful manufacture of pipe, particularly large diameter pipe, the PE resins should have sufficiently low viscosities at high shear rates to facilitate extrusion but sufficiently high viscosity at low shear rates to minimize gravitational flow (slump or sag) of the extruded profile before it has sufficiently cooled and solidified.
Resins useful for pipe applications having broad molecular weight distributions are disclosed in U.S. Pat. Nos. 6,525,148 and 6,867,278. The resins are produced using a catalyst system comprising a chromium source on an aluminophosphate support.
PE resin compositions comprised of relatively higher and lower molecular weight components and having a bimodal (BM) molecular weight distribution (MWD) have been disclosed for pipe applications. Such resins, produced using various tandem reactor polymerization processes, have an acceptable balance of strength, stiffness, stress crack resistance and processability as a result of the contributions of the different molecular weight PE species. For a general discussion of bimodal resins and processes see the articles by J. Scheirs, et al., TRIP, Vol. 4, No. 12, pp. 409-415, December 1996 and A. Razavi, Hydrocarbon Engineering, pp. 99-102, September 2004. Bimodal processes are also discussed in the article by R. Scherrenberg, et al., “Product Optimization by Full Exploitation of the Intrinsic Flexibility of Bimodal Processes” poster paper presented at Plastic Pipes XII, Milan, Apr. 19-22, 2004.
EP 1201713 A1 describes a PE pipe resin comprising a blend of high molecular weight PE of density up to 0.928 g/cm3 and high load melt index (HLMI) less than 0.6 g/10 min and lower molecular weight PE having a density of at least 0.969 g/cm3 and MI2 greater than 100 g/10 min. The resin blends which have a density greater than 0.951 g/cm3 and HLMI from 1-100 g/100 min are preferably produced in multiple reactors using metallocene catalysts.
U.S. Pat. No. 6,252,017 describes a process for copolymerizing ethylene in first and second reactors utilizing chromium-based catalyst systems. Whereas the resins have improved crack resistance they have a monomodal MWD.
U.S. Pat. No. 6,566,450 describes a process wherein multimodal PE resins are produced using a metallocene catalyst in a first reactor to obtain a first PE and combining said first PE with a second PE of lower molecular weight and higher density. Different catalysts may be employed to produce the first and second PEs.
U.S. Pat. No. 6,770,341 discloses bimodal PE molding resins with an overall density of ≧0.948 g/cm3 and MFI190/5≦0.2 g/10 min. obtained from polymerizations carried out in two successive steps using Ziegler-Natta catalysts.
Multi-modal PEs produced by (co)polymerization in at least two steps using Ziegler-Natta catalysts are also disclosed in U.S. Pat. No. 6,878,784. The resins comprised of a low MW homopolymer fraction and a high MW copolymer fraction have densities of 0.930-0.965 g/cm3 and MFR5 of 0.2-1.2 g/10 min.
U.S. Pat. No. 7,034,092 relates to a process for producing BM PE resins in first and second slurry loop reactors. Metallocene and Ziegler-Natta catalysts are employed and in a preferred mode of operation a relatively high MW copolymer is produced in the first reactor and a relatively low MW homopolymer is produced in the second reactor.
U.S. Pat. Nos. 6,946,521, 7,037,977 and 7,129,296 describe BM PE resins comprising a linear low density component and high density component and processes for their preparation. Preferably the resin compositions are prepared in series reactors using metallocene catalysts and the final resin products have densities of 0.949 g/cm3 and above and HLMIs in the range 1-100 g/10 min.
BM PE resins comprised of low molecular weight (LMW) homopolymer and high molecular weight (HMW) copolymer and wherein one or both components have specified MWDs and other characteristics are described in U.S. Pat. Nos. 6,787,608 and 7,129,296.
U.S. Pat. No. 7,193,017 discloses BM PE compositions having densities of 0.940 g/cm3 or above comprised of a PE component having a higher weight average MW and a PE component having a lower weight average MW and wherein the ratio of the higher weight average MW to lower weight average MW is 30 or above.
U.S. Pat. No. 7,230,054 discloses resins having improved environmental stress crack resistance comprising a relatively high density LMW PE component and relatively low density HMW PE component and wherein the rheological polydispersity of the high density component exceeds that of the final resin product and the lower density component. The resins can be produced by a variety of methods including processes utilizing two reactors arranged in series or in parallel and using Ziegler-Natta, single-site or late-transition metal catalysts or modified versions thereof. Silane-modified Ziegler-Natta catalysts are used to produce the narrower polydispersity lower density component.
Copending application Ser. No. 12/156,844, filed Jun. 5, 2008, discloses bimodal PE resins having improved SCG and RCP resistance by virtue of their reduced long-chain branching and a process for their preparation. The improved resins are obtained using a two-stage cascade polymerization process utilizing a high activity Ziegler-Natta catalyst system and alkoxysilane modifier.
There is a continuing need in the industry for resins that have an improved balance of properties suitable for pipe applications. There is a particular need for multimodal resins having good SCG and RCP resistance and improved melt strength suitable for the production of thick-walled pipe and for processes for making such resins utilizing Ziegler-Natta catalysts.